Edible oils, including vegetable and animal fats and oils, typically contain free fatty acids (FFAs), small amounts of mono-glycerides and di-glycerides and other undesirable contaminants. Not all contaminants are necessarily toxic. Most edible oils also contain heavy metals, furans, such as polychlorinated dibenzofurans (PCDFs), dioxins, such as polychlorinated dibenzo-p-dioxins (PCDDs), and polychlorinated biphenyls (PCBs). Depending on the type of oil and its application, a number of different processing steps may be employed to refine the oil to meet desired specifications.
Conventional edible oil refining has been long practiced and is well documented in numerous publications, such as “Bailey's Industrial Oil and Fat Products,” 6th Edition, Fereidoon Shahidi (editor), ISBN 978-0-471-38460-1, April, 2005, as well as other notable handbooks in the fats and oils industries. Regardless of the type of oil and its application, the final step in the refining process is generally deodorization, which is the removal of FFAs to meet residual odor specifications of the final product.
Depending on the type of oil and its intended use, fatty acids are typically reduced to about 2% by weight or less, and more commonly to less than about 1%. This is conventionally accomplished by first degassing at moderate vacuum, typically between about 50 and 150 Torr at about 250° F. or so, to eliminate moisture, oxygen and other light-ends, heating the oil to stripping temperatures, and pumping the heated oil to a deodorizer operating under deep vacuum to minimize operating temperature. However, in order to remove the odor to acceptable levels, stripping steam, typically about 1% by weight is introduced at the base of the deodorizer to aid in stripping the FFAs. In most applications, such as soybean or canola oil, the deodorization is not complete unless the oil is held at operating temperatures for about 15 to 30 minutes, with direct steam sparge injection in a holding zone to complete the deodorization.
However, animal fats and oils, in particular marine oils, are prone to degradation and loss of desirable omega-3 fatty acids, such as docosahexaenoic acid (DHA) and eicoapentaenoic acid (EPA), during the deodorization step. Therefore, lower temperatures and deeper vacuums, in the micron range, are generally employed to preserve the desirable omega-3 fatty acids, while removing a significant percentage of the FFAs and other undesirable contaminants. While conventional deodorization has been effective in reducing the FFAs with no appreciable loss of the omega-3 fatty acids, the same cannot be said about the removal of PCBs and dioxin-like PCBs. These compounds are prohibited in aquaculture, pet foods, nutritional supplements and pharmaceutical applications, with very strict limits on some of the toxins to parts-per-billion (PPB) or, in most cases, to a few parts-per-trillion (PPT) for food and nutraceutical applications. For fish oil, standards have been adopted by the World Health Organization (WHO) and by the Global Organization for EPA and DHA Omega-3 s (GOED), which is a trade association representing many companies worldwide active in the EPA and DHA omega-3 industry.
Presently, in order to remove the PCBs to these low levels, while preventing degradation and loss of the valuable omega-3 fatty acids, short path (SP) or molecular distillation has been employed. (Breivik, H & Thorstad, O., “Removal of organic environmental pollutants from fish oil by short-path distillation,” Lipid Technol., Vol. 17, pp. 55-58, January, 2005.) However, these technologies present a number of disadvantages, including the normal practice of bleaching and winterization prior to distillation, and most importantly loss of the product during distillation, reportedly in the range of about 4-5%, and as high as about 15%, depending on the desired level of residual toxic contaminants in the product. The reason for this rather significant loss of the product is the severe operating conditions, including operating pressures in the micron range of vacuum, such as 0.05 Torr, and temperatures exceeding 400° F., and more typically 430° F. depending on the specifications of the product.
Short path or molecular distillation are single-stage devices. Thus, in order to meet the residual contaminant concentration requirements, the product must be processed by several such devices in series, or the product must be recycled several times through the equipment to simulate additional equilibrium stages. This practice is costly and inefficient, both in terms of energy consumption and processing time, in addition to exposing the product to high temperatures longer than ideally desired, which can cause darkening of the oil.
In addition, it has been reported that after the removal of PCBs by molecular distillation, the oil is still required to go through a deodorization process to remove remaining odorous materials, i.e., the FFAs and flavor compounds. See, for example, U.S. Pat. No. 4,996,072, the entire contents of which are hereby incorporated by reference herein, for all purposes.
Other disadvantages of molecular distillation include its high initial capital and installation costs, including cost of an appropriate vacuum system, and high operating costs associated with the relatively large number of moving parts, such as high-speed rotors, seals, bearings, etc.